SAN FRANCISCO (CBS SF) — Summer has officially ended with the first day of fall arriving Saturday.

In San Francisco, the 2012 Autumn Moon Festival is marking the  start of a new season Saturday and Sunday with the San Francisco Chinatown  Merchants Association presenting live entertainment, food, drinks, cultural  booths, arts and crafts, and a parade in Chinatown along Grant Avenue.

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Festival organizers said the celebration showcases Chinese culture  during the brightest and fullest moon on the lunar calendar, which coincides  with the autumnal equinox on the Western calendar.

The festival will run throughout the weekend between 11 a.m. and 6  p.m.

To celebrate the changing of the seasons, students at Moraga’s  Saint Mary’s College are putting their studies to use.

The start of fall also marks the autumnal equinox when the amount  of day and night is equal, and at 1 p.m., or solar noon, students in the  college’s “Great Books” program who have learned astronomy from ancient Greek  texts will mark the equinox using ancient astronomical measurement tools,  including a “plinth” and a “gnomon.”

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A plinth is a rectangular slab of granite that helps measure the  elevation of the sun above the horizon. A gnomon is traditionally part of a  sundial and the student’s 6-foot tall concrete structure will cast a shadow  that will allow students to track the sun’s movement.

The autumnal celebration will be held at the college located at  1928 St. Mary’s Road in Moraga.

According to the National Weather Service, weather for the first  day of fall in the Bay Area has been mild with mostly clear skies expected  throughout the day.

Temperatures may dip into the 40s overnight in some valley areas,  while daytime temperatures are in the high-60s along the coast and pushing  into and beyond the upper-70s inland.

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